In double rail traction return systems, the traction current return is through both running rails in parallel. The rails may be continuous, or divided into sections for the purpose of determining train position. In the latter case the rail sections are separated by insulating block joints to isolate signal currents to particular sections. The insulating block joints are then bypassed for traction currents by impedance bonds having low impedance at the traction frequency. These bypass impedance bonds consist of (transverse) impedances connected between the rails on each side of the insulating block joint, centre taps of the two transverse impedances being commoned to bypass the insulating block joints.
In the case of continuous rails, impedance bonds between the rails are used to equalise the traction return currents at intervals. These impedance bonds are centre tapped and connected to a return conductor which is earthed and connected to support structures for the `live` conductor of the traction supply.
For the purpose of determining the position of a train on the rails, A.C., particularly audio frequency, signals are fed along sections of the track from a transmitter connected between the rails to a tuned receiver similarly connected between the rails at a distance of the order of 1 km. A train within that track section, i.e. between the transmitter and receiver, will provide a sufficiently low impedance short circuit to short out the track signal before it reaches the receiver. A track relay held by the receiver when energised drops out to indicate occupancy of the section.
There are commonly several impedance bonds to each track circuit although they are normally positioned independently of the transmitter.
The presence of the above impedance bonds in double rail return systems causes difficulty in detecting a break in one of the rails. If a rail break occurs on the receiver side of an impedance bond the broken rail between that impedance bond and the next one in the receiver direction is in effect replaced by the earthed conductor between the centre taps of the two impedance bonds. In addition, the two impedance bonds, which are basically inductance coils, act as step-down and step-up auto-transformers respectively, so that a substantial part of the audio signal appears across the second impedance bond, and thus across the receiver.
Such a fault may very well not prevent the detection of a train in the track section since the train will tend to short circuit either the transmitter or the receiver according to the position of the train in relation to the break in the rail.
Similar remarks apply to an impedance bond with an "open" fault.
Consequently, it may be seen that a broken rail may go undetected until a derailment occurs.
An object of the present invention is therefore to provide a detection circuit for a double rail return system, capable of detecting a break in a rail or an open bond despite the bypassing effect of transverse impedance bonds.